Considering the case of a surface with a cavity recessed in the surface, when the surface, and hence the recessed cavity, is moving in ambient fluid, for example when an aircraft with an open bay, for example an open weapons bay or an open undercarriage bay, is moving in air, a shear layer is formed between (i) the moving ambient air that is flowing across the surface and across the top of the recessed cavity, and (ii) the static air in the cavity (from the reference point of the aircraft). A vortex is shed from the cavity leading edge and grows as it travels down the shear layer and impacts on the aft (trailing) wall of the bay resulting in the emission of noise. Also the acoustic wave travels back upstream inside the bay. The fluctuating pressure of the acoustic wave may either result in vortices being shed from the leading edge cavity lip or an increase in the growth rate of the vortices such that a series of vortices is formed down the shear layer at a preferential rate which is related to the frequency of the upstream acoustic wave. The vortices grow into large scale structures as they propagate downstream in the shear layer and then impact the aft (trailing) wall of the bay at a characteristic rate. This results in acoustic noise being generated at a characteristic rate which may be described as acoustic tones of a characteristic frequency.
The frequency of the tones may be formulated using Rossiter's equation. It can be seen that there is a feedback loop formed by the passage of the vortices and the upstream propagating acoustic wave.
U.S. Pat. No. 5,699,981 discloses an aircraft cavity acoustic resonance suppression system which comprises a small diameter, substantially cylindrically shaped member disposed substantially parallel to and spaced up to a distance corresponding to about three airflow boundary layer thicknesses from the surface of an aircraft near the leading edge of the cavity and transverse to airflow thereacross. An actuator is provided to select the adjustment of the spacing between the member and the aircraft surface according to different operational speeds and hence different operational boundary layer thicknesses as the member's spacing of about three airflow boundary layer thicknesses from the surface of the aircraft is disclosed as critical.
More generally, air intakes or other tunnel like arrangements are known. Such arrangements, even if considered as including a cavity as such, do not include cavities that are of the type being addressed in the present invention, which on the contrary include cavities recessed in a surface.